Holocene palaeomagnetic secular variation at Lake Biwa, central Japan

We have conducted a palaeomagnetic study on three sediment cores obtained from a single site in Lake Biwa, central Japan. A total of 1430 discrete samples from three cores show strong and stable remanence, which is carried by pseudo-single-domain magnetite. Palaeomagnetic records from individual cores show little scatter of directions, and there is excellent correlation among the records from the site. A composite palaeomagnetic secular variation (PSV) record for the Holocene was constructed after fine-scale adjustment and stacking of the three records. The age model is based on two wide-spread tephra layers: Kawagodaira (3150 yr BP, calendar years) and Kikai-Akahoya (7250 yr BP). Our results show that the declinations and inclinations are strikingly similar to archaeomagnetic data from southwest Japan for the last 2000 years. The PSV record from Lake Biwa shows good correlation in directions with that of Beppu Bay, located about 500 km west of Lake Biwa, although the Beppu Bay record shows relatively smaller amplitudes of both inclination and declination, probably due to heavy smoothing of the initial results. When the Lake Biwa record is compared with the proposed type curve for southwest Japan, there is considerable disagreement throughout the Holocene. A re-analysis of the data sets and other information used to construct the type curve revealed that the discrepancy in the results was due to inadequate age determination for the cores used in previous studies. We conclude that the stacked PSV record from Lake Biwa meets the criteria required for a reliable curve. Moreover, our results resolve the complications found in the correlation of PSV records in southwest Japan and thereby contribute to a better understanding of Holocene PSV.